1. Field of the Invention
The present invention relates generally to optical fibers, and particularly to optical fibers having a relatively large amount of small-angle scatter, and sensors using same.
2. Technical Background
Optical fibers were developed some thirty years ago for long-distance telecommunication because of their large bandwidth, light weight, immunity from electromagnetic interference, and other advantageous properties. The typical optical fiber has a high-refractive-index core region surrounded by a low-refractive-index cladding. A protective coating is usually provided over the cladding to protect the structure from the environment. Light injected into the optical fiber travels down the core region of the optical fiber, with a small portion of the light propagating in the cladding as an evanescent field (“evanescent wave”). The evanescent wave typically penetrates a short distance into the cladding (e.g., a few microns), depending on the optical fiber's refractive index profile.
The distance over which optical signals can be transmitted over an optical fiber is limited by attenuation (loss) due to absorption and scattering (e.g., Rayleigh, Brillouin and weak scattering), as well as from geometric effects (e.g., bending). Consequently, over the years, tremendous effort has been directed to studying and reducing these sources of signal attenuation.
More recently, optical fibers have been used in sensors for measuring a host of different physical properties such as temperature, pressure, strain, chemical composition, current/voltage, vibration/acceleration, and absorbance/fluorescence. One type of optical fiber sensor is referred to as an “evanescent optical fiber sensor.” In a typical evanescent optical fiber sensor, light from the optical fiber core is transferred to the test medium via optical coupling of the evanescent wave with the test medium. The change in the amount and/or wavelength of the detected light is then translated into a measurement of the test medium.
Typically, the sensitivity of an evanescent optical fiber sensor is improved by transferring more light from the core to the test medium beyond that naturally conveyed by the evanescent wave. For example, increasing the strength of the evanescent wave penetrating into a biological sample to be assayed increases the amount of fluorescence, which translates into improved sensor performance. The need for improved evanescent optical fiber sensors is growing due to the increasing demand for measurement sensitivity.